1. Field of the Invention
The present invention relates to a synchronous motor utilizing a permanent magnet.
2. Description of the Related Art
One type of conventional synchronous motor is a permanent magnetic synchronous motor such as shown in FIG. 1. Such a motor has a permanent magnet PM1 with a N pole arranged radially outward and a permanent magnet PM2 with a S pole arranged radially outward. The motor shown has a rotor shaft 1, a rotor yoke 2, and a stator (not shown) which is of a type commonly used in a three-phase induction motor, or the like.
A known example of the above conventional motor would be a permanent magnetic motor having an embedded magnet structure, such as is disclosed in such as is disclosed in Memoir D by Institute of Electrical Engineers of Japan, Vol. 114, Issue 6, 1994, pp. 668 to 673, "Wide Range Variable Speed Control for a PM Motor with Embedded Magnetic Structure", and so on.
FIG. 2 shows an example of a three-phase and six-pole synchronous reluctance motor, provided with a thin magnetic flux path 14 for magnetically connecting magnetic poles, and a slit 10, which is either a space or made of non-magnetic member, formed between the magnetic flux paths 14. The motor also has a rotor shaft 1, a rotor yoke 13, and a link 15 in a radial direction. The link 15 is not just unnecessary from a magnetic point of view, its presence can be harmful in light of an electromagnetic operation of the motor as leakage flux passes therethrough. Nevertheless, the link 15 is required to mechanically connect the rotor yoke 13 and each magnetic flux path 14 for structural reinforcement. The link 16 on the external circumference of the rotor similarly reinforces the rotor as entirety. The rotor has a structure in which flat rolled magnetic steel sheets and strips, each having the shape as shown in FIG. 2, are laminated in the direction of the rotor shaft. The stator 12 has slots where an three-phase six-pole AC winding passes.
Operation of the synchronous reluctance motor FIG. 2 will be described referring to FIG. 3, which shows a modeled two-pole synchronous reluctance motor, provided with thin magnetic flux paths NMP and slits SG. A magnetic flux path NMP is a path where magnetic flux passes from one magnetic pole to another. A slit SG is a space formed between adjacent thin magnetic flux paths NMP.
The rotor of FIG. 3 has a structure in which smaller magnetic resistance is caused in the vertical (d-axis) direction of the rotor and larger magnetic resistance is caused in the horizontal (q-axis) direction. A stator 7 is also shown in the drawing.
When the rotor is excited by magnetizing current id, N and S poles are formed as indicated in the figure, thereby creating a magnetic flux MFd. When a torque current iq is then supplied in the direction of the magnetic flux MFd, force F1 is caused. As the torque current iq additionally causes a magnetic flux MFq, force F2 is thus caused which is proportional to a product of the magnetizing current id and the magnetic flux MFq. As a result, the motor generates a rotation torque which is proportional to the force (F1-F2).
The above operation of FIG. 3 can be expressed using vectors, as shown in FIG. 4, in disregard of losses, such as winding resistance, leakage inductance, core loss, and soon, of the motor. Current i0, or an added current of the magnetizing current id and the torque current iq, is supplied to the motor. When the motor rotates at a rotation angle frequency .omega. with d-axis inductance Ld and q-axis inductance Lq, a voltage Vd=-Lq.multidot.diq/dt=-.omega.Lq.multidot.iq will be caused in the direction of the flow of magnetic current id, while a voltage Vq=Lq.multidot.did/dt=.omega..multidot.Ld.multidot.id will be caused in the direction of the flow of torque current iq. voltage V0 is an added voltage of the voltages Vd and Vq. Motor output power P is expressed as P=.omega..multidot.Ld.multidot.id.multidot.iq-.omega..multidot.Lq.multidot .iq.multidot.id=.omega..multidot.(Ld-Lq).multidot.id .multidot.iq=v0.multidot.i0.multidot.COS(.theta.PR), in which .theta.PR is a phase difference between voltage V0 and current i0, and COS(.theta.PR) is a power factor.
FIG. 5 is a longitudinal cross sectional view of a three-phase six-pole synchronous motor of a hybrid type which has a pair of motors using permanent magnets and a pair of field winding. FIG. 6A is a lateral cross section of the rotor of FIG. 5 along the line EF; FIG. 6B is a lateral cross section of the same along the line GH. A three-phase AC winding 28 passes through the respective stators ST1, ST2 (25, 26) of the two respective motors, winding thereabout in the same manner as a three-phase AC winding of a typical three-phase inductance motor does. A field winding 29 winds around the stator in the rotor rotation direction, and excites the magnetic flux, passing through the stators and rotors as indicated by the arrow 30, of a magnetic field. A rotor shaft 1 is also shown in the drawing. A permanent magnet 22 constitutes a N pole of the rotor RT1 on the left side. Three permanent magnets 22 are provided each for every electrical angle of 360.degree. in the rotor rotation direction. The rotor RT1 has a magnetic flux path 23. A permanent magnet 32 constitutes a S pole of the rotor RT2 on the right side. Three permanent magnets 32 are provided for every electrical angle of 360.degree. in the rotor rotation direction at a position differing from that of each permanent magnet 22 by an electrical angle of 180.degree. in the rotor rotation direction. Back yokes 24 and 27 on the rotor and stator sides, respectively, induce magnetic flux in the direction of the rotor shaft.
Magnetic flux in the magnetic poles 31, 21, which are made of soft magnetic material, varies due to the current flowing in the field winding 29. Specifically, when the magnetizing current IFS for the field winding 29 is zero (IFS=0), the magnetic flux is not excited on the magnetic poles 31, 21, and instead is formed between the permanent magnets 22 and 32. When the magnetizing current IFS is negative, provided that a magnetic flux is caused in the direction with the arrow 30, the magnetic pole 31 is rendered to be a N pole, while the magnetic pole 21 is rendered to be a S pole. The magnitude of the magnetic flux is proportional to that of the field magnetizing current IFS. When the magnetizing current IFS is positive, field magnetic flux is caused in the direction opposite from that with the arrow 30. As a result, the magnetic pole 31 is rendered a S pole, while the magnetic pole 21 is rendered a N pole. The magnitude of the magnetic flux is proportional to that of the field magnetizing current IFS. When the magnetizing current IFS is positive, respective magnetic poles of the rotor RT1 and of the rotor RT2 are alternately rendered to be N and S poles in the rotor rotation direction, and the motor resultantly functions like a permanent magnet synchronous motor, as shown in FIG. 1. When the magnetizing current IFS is negative, on the other hand, the magnetic poles of the rotor RT1 all serve as a N pole, so that a difference in magnetic flux between the magnetic poles 22 and 31 resultantly serves as a function of the motor, producing an effect similar to flux-weakening. Meanwhile, the respective magnetic poles of the rotor RT2 all serve as a S pole, so that a difference in magnetic flux between the magnetic poles 32 and 21 resultantly serves as a function of the motor, achieving an effect similar to flux-weakening. As described above, by controlling the magnetizing current IFS by varying in a range between positive and negative, effective magnetic flux of a magnetic field can be strengthened or weakened. This enables rotation frequency control of the synchronous motor in a wider range.
Although permanent magnet synchronous motors such as shown in FIG. 1 are widely used because of their easiness of torque control, they have a problem of incapability of constant power control through flux-weakening control when it rotates at base rotation frequency or larger rotation frequency as magnetic field is substantially fixed due to the permanent magnet property, ruling out application of flux-weakening control.
Although a permanent magnet motor with an embedded magnet is capable of flux-weakening control by invertedly exciting a magnetic field flux, it has a problem of deteriorated efficiency in light load driving at high speed rotation as the motor always requires large current for flux-weakening control. The motor has another problem that it requires a safety device to separate a motor and a control circuit for safety at the time of emergency, such as power failure, occurring during high speed rotation as flux-weakening control cannot then be applied and the motor thus generates large voltage.
A synchronous reluctance motor, as shown in FIGS. 2 and 3, also has a problem that, due to generation of magnetic flux MFq by torque current iq, a force F2 proportional to a product of the magnetizing current id and magnetic flux MFq is caused in a direction opposite from that of an output torque, and motor power is thus deteriorated. The deterioration leads to deteriorated motor efficiency and power factor.
A synchronous motor of hybrid exciting type, as shown in FIG. 5, is capable of substantially ideal field control with a small torque current. However, in actuality, reaction of an armature will increase when a torque current is supplied, and magnetic flux distribution in the magnetic poles 31 and 21 is thereby distracted in the rotation direction, compared to a case of a torque current being zero. In particular, when the motor rotates at a high speed, such as at base rotation frequency or larger rotation frequency, unfavorable drop in torque generation and/or unfavorable increase in terminal voltage of the synchronous motor may be caused.